The invention relates to an optically variable security device for use, for example, for securing documents and articles against counterfeiting.
The use of optical thin film structures to inhibit counterfeiting and illicit tampering is now a well established art. See for example U.S. Pat. No. 3,858,977, U.S. Pat. No. 4,721,217, U.S. Pat. No. 4,930,866, U.S. Pat. No. 4,994,314, U.S. Pat. No. 5,009,486, EP-A-0733919 and Dobrowolski, J. A, Ho, F. C., and Waldorf, A. J. “Research on Thin-Film Anticounterfeiting coatings at the National Research Council of Canada,” Applied Optics, Vol 28, No 14, pp 2702–2717, 1989. These thin film structures often referred to as optical multilayers generally fall into two types; those which are purely dielectric and those which are composed of alternate dielectric (metal oxide or polymer) and metallic layers. The former has the advantage of being transparent and therefore suitable for over-laminate applications, however it requires a minimum of five layers to produce an iridescent device with acceptable luminosity and colour saturation (seen to best effect when located over a dark coloured substrate or background). Whereas the metallic combination functions well with only 3 layers and is therefore significantly cheaper to fabricate. However in both cases the iridescent quality of the multi-layer is strongly influenced by the deposition technology.